A rotating body such as a shaft that transmits a torque or the like may vibrate due to fluctuations in an input torque itself, fluctuations in a load torque resulting from a load member that is coupled to the rotating body, a slight deviation of the center of gravity of a rotary system, or the like. Such torque fluctuations act on the rotating body as torsional vibrations. Various devices for reducing the torsional vibrations have been conventionally developed. One such example is a device that is referred to as a so-called dynamic damper. The dynamic damper is configured to form another vibration system other than the rotating body by an inertia mass body that is separately provided, and to reduce vibrations with a frequency equivalent to a resonant frequency of the vibration system caused by the inertia mass body, due to the fact that the resonant frequency of the vibration system by the inertia mass body is different from the resonant frequency of the rotating body.
In this kind of device, the inertia mass body vibrates relatively to the rotating body, and hence friction occurs at contact locations therebetween. If the frictional force or the friction resistance is large, abrasion progresses to cause a fall in durability. Besides, the frictional force acts in such a manner as to suppress vibrations of the inertia mass body. Therefore, if the frictional force changes, the frequency characteristic changes, and it may become impossible to reduce the target vibrations. Besides, on the other hand, the torsional vibrations that are transmitted from an internal combustion engine differ depending on the rotational speed. Therefore, a vibration damping device that has a natural frequency adapted for the rotational speed may be employed for the internal combustion engine. Such a so-called rotational speed adaptation-type vibration damping device is configured, as an example, to change the pendulum radius of the inertia mass body that makes pendular movements.
A device that is configured to reduce friction between the inertia mass body and the rotating body as the former is described in Japanese Patent Application Publication No. 2002-340097 (JP-2002-340097 A). In the device described in this Japanese Patent Application Publication No. 2002-340097 (JP-2002-340097 A), a rigid body pendulum is rotatably attached, by a spindle, to an outer peripheral side that is offset from a rotation center of a pulley body that is integrated with a drive shaft. Abrasion reduction means such as a fluorocarbon resin coat or the like is provided between an outer peripheral surface of the spindle and an inner peripheral surface of a hole that is penetrated by the spindle. Besides, a rotational speed adaptation-type vibration damping device as the latter is described in Published Japanese Translation of PCT Application No. 2011-504987 (JP-2011-504987 A).
The aforementioned device described in Japanese Patent Application Publication No. 2002-340097 (JP-2002-340097 A) can reduce friction between an inertia mass body and a rotating body to suppress abrasion and enhance durability. However, since the pendulum radius of the inertia mass body is constant, the natural frequency changes depending on the rotational speed. In the case where the frequency of vibrations to be damped changes as a result, it may not be possible to obtain desired vibration damping characteristics. Accordingly, in the case where the rotational speed of the rotating body as a subject of vibration damping changes and the order of vibrations to be damped changes as a result, a device structured to change the substantial pendulum radius as described in Published Japanese Translation of PCT Application No. 2011-504987 (JP-2011-504987 A) is employed. In that case, as described in Published Japanese Translation of PCT Application No. 2011-504987 (JP-2011-504987 A), a rolling contact surface whose curvature is constant or changes is formed on at least one of the rotating body and the inertia mass body, and the inertia mass body or a shaft member that removably supports this inertia mass body is supported by the rolling contact surface and rolled along the rolling contact surface. That is, the rolling contact surface serves as a contact location between the inertia mass body and the rotating body.
In the vibration damping device that is configured to roll the inertia mass body, desired vibration damping performance is exerted through smooth rolling of the inertia mass body (i.e., a rolling element). However, the rolling contact surface is a curved surface that has a center of curvature at a location spaced apart from the rotation center of the rotating body. Therefore, in a state where the rolling element moves to a position that is offset from a line that links the center of the rotating body with the center of curvature of the rolling contact surface, the load in the direction of a normal line, which presses the rolling element against the rolling contact surface as a result of a centrifugal force, is small, and a large force is applied in a tangential direction. Accordingly, if a treatment of reducing friction as described in Japanese Patent Application Publication No. 2002-340097 (JP-2002-340097 A) is applied to the rolling contact surface or a predetermined outer surface of the rolling element that is in contact therewith, the rolling element slips with respect to the rolling contact surface without rolling thereon. As a result, desired vibration damping performance may not be obtained.